1. Technical Field
This invention relates to semiconductor memories or arrays and more particularly to a stacked double density memory module which uses industry standard memory chips.
Integrated circuit semiconductor memory chips are widely employed in data processing systems ranging from inexpensive home or personal computers to large mainframe systems. Integrated circuit memory chips, hereinafter referred to as memory chips or chips, comprise a packaged (encapsulated) semiconductor memory array, provided with a plurality of input/output pins. Presently, read/write memory chips, also referred to as Dynamic Random Access Memories (DRAMs), are available from many suppliers worldwide. The package dimensions and pin assignments for all memory chips having the same bit storage capacity are standardized, so that memory chips of the same bit storage capacity are interchangeable, regardless of the supplier.
One commonly used memory chip is the 64K DRAM. The 64K DRAM stores approximately 65,000 bits of data and is widely available in dual-in-line plastic, ceramic or plastic/ceramic packages. Regardless of the supplier, all 64K DRAMs are interchangeable in that the package dimensions and pin assignments are standardized. In particular, the industry standard 64K DRAM is available in a 16 pin package having the following pin assignments (with the pins being numbered counterclockwise from the upper left corner of the chip):
______________________________________ Pin Number(s) Pin Function ______________________________________ 1 No connect - unused 2 Data Input 3 Write Enable 4 Row Address Select-Chip Enable 5-7, 9-13 Address 8, 16 Power Supply 14 Data Output 15 Column Address Select ______________________________________
2. Background Art
In the quest for ever increasing storage density, computer designers have realized that stacked memory chips may be employed to double the storage density on a given printed circuit board. More particularly, since corresponding address, power supply, and data lines may be paralleled, two memory chips can be physically stacked upon one another with the bottom chip mounted on a printed circuit board or other second level package. Separate chip enable pin locations must be provided for each chip in order to select either the top or bottom chip. When stacked memory modules are employed, the memory storage density per printed circuit board unit area is effectively doubled. With this advantage in mind, the art has employed two basic approaches for stacking memory chips while still providing separate chip select pin locations for the top and bottom chips:
1. Larger Printed Circuit Board Footprint: Since two chip select paths are required at every stacked chip location on the circuit board, the chip select pins from the upper and lower chips may be routed to two separate printed circuit board locations. Thus, for example, two standard 16 pin 64K DRAMs may be mounted in an 18 pin socket, with the socket providing separate wiring paths for the chip enable pins from the top and bottom chips. Such an approach clearly wastes printed circuit board space because an 18 pin footprint is required for stacking two 16 pin chips. Moreover, a connector or housing for mounting the stacked DRAMs is generally required in order to provide the wiring paths from 16 pins to 18 pins. A connector or housing also wastes space on the printed circuit board, increases cost, and decreases the reliability of the stacked module, thereby offsetting some of the advantages of stacked chips.
2. Customized Pin Assignments: The pin assignments of one or both of the chips may be rearranged from the industry standard to permit stacking. Thus, for example, in the 64K DRAM industry standard pin assignment, pin 1 is a no-connect (unused) pin and pin 4 is the row address select (chip enable) pin. Customized 64K DRAM chips may be procured wherein pin 1 is the row address select (chip enable) pin and pin 4 is a no-connect (unused) pin. Then, an industry standard and customized 64K DRAM may be stacked. While such an approach does provide a 16 pin footprint on the printed circuit board, it will be understood that customized chips are much more expensive than industry standard chips because they are not available as off-the-shelf items. Accordingly, the increased cost of the non-standard chip offsets some of the advantage of stacked chips.
In conclusion, although the 64K DRAM is widely used in personal, mini, and mainframe computers, and in non-computer oriented products such as video games, it has not been heretofore recognized that two 64K DRAMs may be stacked to obtain a double density memory module, without increasing the printed circuit board footprint size, without requiring the use of a socket or housing and without requiring customized pin assignments.